U.S. county home sales volume — past 10 years + forecast
Each arrow is a U.S. county. Angle and color encode the 12-month percent change in monthly sales count (Zillow); longer arrows mean larger swings. The slider covers 120 historical months (Mar 2016 → Feb 2026) followed by a 12-month forecast (Holt's damped-trend exponential smoothing); forecast frames are drawn with reduced opacity. The fan chart below the map shows the national outlook with 50 / 80 / 95% prediction intervals.
How to read this map
Each arrow sits at the centroid of one U.S. county. Its angle reports the sign of the year-over-year change in monthly sales count — pointing sharply upward for a big increase, downward for a big decline, near-horizontal when transactions are flat. Color and length redundantly encode magnitude.
The slider walks through 120 historical months (10 years) and then 12 forecast months; arrows in the forecast region are drawn at lower opacity and the timeline track switches to a dashed segment.
Forecast model
Per-county point forecasts come from Holt's exponential smoothing with damped trend (statsmodels). The smoothing-trend coefficient is bounded to (0, 0.05) and damping to (0.5, 0.85) — without those bounds the optimiser reads a recent monthly dip as a permanent slope and projects -38% by month 12. Constrained, the forecast asymptotes near the smoothed level.
The fan chart's bands come from 500 stochastic simulations of the fitted national model (mean YoY across counties), with quantiles at 2.5/10/25/50/75/90/97.5%. They are model intervals — they capture residual variance, not regime change.
Data & method
Volume series is Zillow's county-level monthly sales count (raw). We apply a 3-month centered moving average to dampen reporting noise, then compute the 12-month percent change against the same month one year earlier. Values are clipped to ±40% for visual encoding; tooltips show the unclipped figure.
Boundaries are U.S. Census TIGER 2022 simplified counties via the us-atlas project, projected with d3.geoAlbersUsa. Counties with sparse Zillow coverage (less than 70% of the 60-month window) are excluded.
